Coenzymes methylcobalamin

RELATIONSHIPS BETWEEN VITAMIN Bj AND FOLIC ACID The major roles of vitamin and folic acid in intracellular metabolism are summarized in Figure 53-6. Intracellular vitamin Bjj is maintained as two active coenzymes methylcobalamin and deoxyadenosylcobal-amin. Deoxyadenosylcobalamin (deoxyadenosyl Bj ) is a cofactor for the mitochondrial mutase enzyme that catalyzes the isomerization of L-methylmalonyl CoA to succinyl CoA, an important... 

METABOLIC FUNCTIONS The active coenzymes methylcobalamin and 5-deoxyadeno-sylcobalamin are essential for cell growth and replication. Methylcobalamin is required for the conversion of homocysteine to methionine and its derivative, SAM. In addition, when concentrations of vitamin Bj are inadequate, folate becomes trapped as methyltetrahydrofolate, causing a functional deficiency of other required intracellular forms of folic acid (see Figures 53-6 and 53-7 and discussion above). The hematological abnormalities in vitamin Bj -deficient patients result from this process. 5-Deoxyadenosylcobalamin is required for the isomerization of L-methylmalonyl CoA to succinyl CoA (Figure 53-6). 

A related coenzyme methylcobalamin resembles Bj2 except for having a CH3 ligand instead of 5 -deoxyadenosyl. Methylcobalamin is involved in certain biological methyl transfers and features homolytic cleavage of its Co—CH3 bond as a key step. 

The finding in 1946 by T. Spies and colleagues that thymine can substitute the functions of folic acid and vitamin B12 led to the understanding that both folic acid and vitamin B12 are involved in methyl transfer reactions (Vilter et al. 1950). Donaldson and Keresztesy showed that folic acid can exist in various forms with one-carbon group attached. The coenzyme methylcobalamin was discovered in 1964 (Lindstrand 1964). 

Metabolism and Mobilization. On entry of vitamin B 2 into the cell, considerable metaboHsm of the vitamin takes place. Co(III)cobalamin is reduced to Co(I)cobalamin, which is either methylated to form methylcobalamin or converted to adenosylcobalamin (coenzyme B>22)- The methylation requires methyl tetrahydrofolate. 

The first two of these are mediated by 5 -deoxyadenosylcobalamin, whereas methyl transfers are effected by methylcobalamin. The mechanism of ribonucleotide reductase is discussed in Chapter 27. Methyl group transfers that employ tetrahydrofolate as a coenzyme are described later in this chapter. 

Lenhert and Hodgkin (15) revealed with X-ray diffraction techniques that 5 -deoxyadenosylcobalamin (Bi2-coenzyme) contained a cobalt-carbon o-bond (Fig. 3). The discovery of this stable Co—C-tr-bond interested coordination chemists, and the search for methods of synthesizing coen-zyme-Bi2 together with analogous alkyl-cobalt corrinoids from Vitamin B12 was started. In short order the partial chemical synthesis of 5 -de-oxyadenosylcobalamin was worked out in Smith s laboratory (22), and the chemical synthesis of methylcobalamin provided a second B 12-coenzyme which was found to be active in methyl-transfer enzymes (23). A general reaction for the synthesis of alkylcorrinoids is shown in Fig. 4. 

In 1933 Challenger et al. discovered that trimethylarsine was synthesized from inorganic arsenic compounds by molds (93). Recently, McBride and Wolfe (94), have reported the synthesis of dimethylarsine from arsenate by cell extracts of the methanogenic bacterium M. O. H. Methylcobalamin is the alkylating coenzyme for this synthesis which requires reduction of arsenate to arsenite, methylation of arsenite to methylarsonic acid, reduction and methylation of methylarsonic acid to dimethylarsinic acid, and finally a four electron reduction of dimethylarsinic acid to dimethylarsine (Fig. 13). 

Figure 15.3 Structural formula of deoxyadenosylcobalamin (coenzyme B, ). (a) A plan view of the corrin nucleus with substituents, (b) The position of the remaining two ligands of the cobalt atom. No attempt is made to show correct stereochemical relationships. Related compounds have different groups in place of the 5 -deoxyadenosyl group cyanocobalamin, (vitamin Bi2)-CN hydroxycobalamin, (vitamin Bi2)-0H methylcobalamin, (vitamin Bi2)-CH3.

In methylcobalamin, X is a methyl group. This compound functions as a coenzyme for several methyltransferases, and among other things is involved in the synthesis of methionine from homocysteine (see p. 418). However, in human metabolism, in which methionine is an essential amino acid, this reaction does not occur. 

The name vitamin B12 indicates a group of cobalt-containing corrinoids, also described as cobala-mins. Hydroxycobalamin (HOCbl), adenosylcobalamin (AdoCbl), and methylcobalamin (MeCbl) are the natural occurring forms. Instead, cyanocobalamin (Figure 19.20) is the commercially available form used for supplements and food fortification, thanks to its greater relative stability. Occasionally, sulfitocobalmin can occur in processed foods. Vitamin B,2 functions as a coenzyme and it is linked to human growth, cell development, and is involved in metabolism of certain amino acids. Vitamin B12 is present mainly in meat and diary foods, therefore a deficiency can occur in... 

Methylbenzene halogen complex of, 3 122 iodine monochloridecomplese, 3 109 Methylchlorosilanes hydrolysis, 42 149-150, 157 pyrolysis products of, 7 356-363 Methylcobalamin, 19 151, 152 Methyl-coenzyme M reductase, 32 323-325 EPR spectra, 32 323, 325 F43 and, 32 323-324 function, 32 324-325 Methyl-CoM reductase, 32 329 Methyl cyanide, osmium carbonyl complexes, reaction, 30 198-201 Methylcyclophosphazene salts, 21 70 synthesis, 21 109... 

Methylcobalamin is the coenzyme form of vitamin It is neurologically active, most bioavailable and best utilized. Unlike cyanocobalamin, it does not require any conversion after absorption by the body and is better retained by the liver and other tissues. It has exhibited beneficial effects against brain aging, irregular sleep patterns. It supports immune function and promote normal cell growth. It represents one of the best values in nutritional products, given its comparably low cost and its wide range of potential benefits. 

Structure of vitamin B12 (cyanocobalamin) and its coenzyme forms (methylcobalamin and 5 -deoxyadenosyl-cobalamin). 

In both bacteria and liver, the 5 -deoxyadenosyl coenzyme is the most abundant form of vitamin B12, while lesser amounts of methylcobalamin are present. Other naturally occurring analogs of the coenzymes include pseudo vitamin B12 which contains adenine in place of the dimethylbenzimidazole. 

The coenzyme evidently functions in a cyclic process. The cobalt alternates between the +1 and +3 oxidation states as shown in Eq. 16-43. The first indication of such a cyclic process was the report by Weissbach that 14C-labeled methylcobalamin could be isolated following treatment of the enzyme with such methyl donors as AdoMet and methyl iodide... 

Figure 16-24 Stereoscopic views of the active site of methionine synthase from E. coli. Methylcobalamin (black) is in the active site with His 759 of the protein in the distal position of the coenzyme in a base-off conformation. The dimethyl-benzimidole nucleotide has been omitted for clarity. Notice the hydrogen-bonded His 759 - D757 -S810 triad. From Jarret et aZ.418a Courtesy of R. G. Matthews.

Model reactions have contributed significantly to our understanding of biological processes. Both pyridoxal phosphate (vitamin B6) and Bi2-coenzymes have proved useful in mechanism studies. Methyl transfer reactions to various metals are of environmental significance. In 1968 it was shown that methylcobalamin could transfer a methyl carbanion to mercury(II) salts in aqueous solutions. Recent research on interaction between B12-coenzymes and platinum salts has shown that charged Ptn salts labilize the Co—-C bond. Secondly, the B12-coenzymes are unstable in the presence of platinum salts this observation correlates with the fact that patients who have received cw-platin develop pernicious anemia. 

Vitamin B12 is a biologically active corrinoid, a group of cobalt-containing compounds with macrocyclic pyrrol rings. Vitamin B12 functions as a cofactor for two enzymes, methionine synthase and L-methylmalonyl coenzyme A (CoA) mutase. Methionine synthase requires methylcobalamin for the methyl transfer from methyltetrahydrofolate to homocysteine to form methionine tetrahy-drofolate. L-methylmalonyl-CoA mutase requires adenosylcobalamin to convert L-methylmalonyl-CoA to succinyl-CoA in an isomerization reaction. An inadequate supply of vitamin B12 results in neuropathy, megaloblastic anemia, and gastrointestinal symptoms (Baik and Russell, 1999). 

Although numerous enzymatic reactions requiring vitamin B12 have been described, and 10 reactions for adenosylcobalamin alone have been identified, only three pathways in man have so far been recognized, one of which has only recently been identified (PI). Two of these require the vitamin in the adenosyl form and the other in the methyl form. These cobalamin coenzymes are formed by a complex reaction sequence which results in the formation of a covalent carbon-cobalt bond between the cobalt nucleus of the vitamin and the methyl or 5 -deoxy-5 -adenosyl ligand, with resulting coenzyme specificity. Adenosylcobalamin is required in the conversion of methylmalonate to succinate (Fig. 2), while methylcobalamin is required by a B12-dependent methionine synthetase that enables the methyl group to be transferred from 5-methyltetrahydrofolate to homocysteine to form methionine (Fig. 3). 

This is another rare inherited disorder of vitamin B12 metabolism in which both coenzyme forms, adenosylcobalamin and methylcobalamin, are affected. Methylcobalamin is required for the transfer of the methyl group of 5-methyltetrahydrofolate to homocysteine to give methionine. Lack of methylcobalamin results in deficient activity of 2V5-methyltetrahydrofolate-homo-cysteine methyltransferase, resulting in a reduced ability to methylate homocysteine. A failure of methionine synthetase would produce a similar result. 

A troublesome feature of this mechanistic interpretation is the absence of direct supporting evidence that the Co-C bond in coenzyme B12 (whose dissociation energy has not yet been determined) is sufficiently weak that facile homolysis under the mild conditions of the enzymic reactions is a plausible process. In fact, alkylcobalamins, including coenzyme B12, exhibit considerable thermal stability and typically do not decompose at measurable rates, in the absence of light or reagents such as 02, until fairly elevated temperatures ( 200°C for methylcobalamin) (4). Among the possible interpretations of this behavior are ... 

Tetrahydrofolate functions as a carrier of one-carbon units. There are numerous metabolic reactions that require either the addition or removal of a one-carbon unit of some specific oxidation state. THF binds one-carbon units of three oxidation levels the methanol, formaldehyde, and formate states. These are shown in Table 6.4 along with their origins and uses. The various one-carbon units are interconvertible, as shown in Figure 6.5. Nicotinamide coenzymes are involved. In addition, the one-carbon unit may be released as C02. The methanol-level THF-bound one-carbon unit 5-methyl-THF is the storage and transport form. Once formed, its main pathway of metabolism is to form methionine from homocysteine, a reaction that requires vitamin B12 in the form of methylcobalamin (see Figure 6.2 and Chapter 20) ... 

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